If April Rogers Farnham had not had to pee at Mile 18 of yesterday’s Boston Marathon, she likely would have finished around the time two explosive devices detonated near the finish line of America’s most historic road race.

Farnham, of Plainfield, was four-tenths of a mile from the end of her first-ever Boston Marathon when race officials and law enforcement stopped runners in the street and told them to stay put.

“It was absolutely scary,” said Farnham during an interview late Monday afternoon. “They stopped us dead. People were just asking questions.”

While she did not hear, see or even smell the devastation that lay ahead, Farnham heard and saw plenty of people who were scared, tired, confused and in shock — either from running for more than four hours or from fleeing the grave scene, where scores of people were injured as a result of the explosions, which were about 15 seconds apart.

“People coming at us told us that bombs had gone off,” she said.

As they waited in the street, Farnham learned that one of the other women who she had been running with on and off throughout the day had a relative who had been injured.

“It was awful,” she said.

All they could do was wait.

“No one had any idea what to do,” she said.

As they waited, runners were throwing up and cramping, exhausted and stressed.

“We were there 40 minutes when all of a sudden people finally started coming out of the buildings, the apartments,” she said. They were offering water, garbage bags to drape over hypothermic runners. Boston residents were handing out food, loaning cellphones, and offering support.

“The people of Boston were awesome,” Farnham said.

Eventually, runners were rerouted, and then moved again to locations to get them closer to where their families and support teams were waiting for them.

Farnham said she was relieved when she found everyone on her team safe and sound.

“There was no way of knowing how anyone was or where they were,” she said. “They are all fine. ... Now we just need to figure out how to get out of the city.”

Farnham had been building toward this race for months.

Through ties to race officials at the Boston Athletic Association, which oversees the marathon, Farnham, a Barre hairdresser who also leads a couple of fitness boot camps and is a fitness consultant, got to wear bib number 25193. While this was not her first marathon, it was Farnham’s first experience at the Boston Marathon.

“I was having a good day,” she said of her time out on the course. “It was amazing.”

At Mile 18, she decided to stop to go to the bathroom.

Given her pace, the weather, the adrenaline, she would have finished close to the tragic moment.

When asked whether she knew how the BAA might handle the incompleted race, she said, “No idea. They have no idea.”

On her Facebook page, Farnham’s friends were swamping her with comments of relief and support.

“I know people were worried,” she said. “I’m OK. We’re OK.”

Hours later, on her site, she wrote, “I feel incredibly blessed.”

Walter Brown of Marshfield, whose grandfather started the BAA, spent the morning at the marathon. As a representative of the family, he traditionally fires one of the starting guns for the race. He was saddened to learn the beautiful day had turned tragic.

“All of us are deeply concerned for the families and people that were injured and killed as a result or the explosions,” he wrote to The Times Argus in an email last evening. “Thanks to the fast response of all support personnel more runners and spectators were kept away from the tragic scene. ... Fortunately, there is a medical facility and numerous medical professionals at the finish line that were able to give immediate attention to the injured.”

Of his friend Farnham, Brown said, “April said she had never experienced anything like the support all of the runners received at this race. ... This speaks to the patriotism, strength, and generosity of spirit of the proud people of Boston. ... I don’t think anyone ever imagined that such a thing would happen at the Boston Marathon and I know Patriots Day events will persevere just as the Patriots have throughout history. We will continue to pray for everyone’s recovery from injury and the health and welfare of all effected by this incomprehensible act.”